The biggest area affected is the fan section. On the PW 2000/4000, the outer fan ring and fan blade tips have been the subject of several ADs. When the 4000 first entered service on the -400, there was substantially much more fan tip erosion than expected, some of this was attributable to FOD, but there was an issue with gyroscopic effects on the N1 section.

On many modern jet engines, esp. the PW4000 series, the reverser cowl supports the engine housing and gives it additional rigidity especially on rotation to prevent it from bending under the gyroscopic forces of the shaft. This feature is called a load sharing cowl and incorporates additional latches. You can´t run the engine with the c-duct open.
If the load sharing latches are not properly adjusted or closed in the correct sequence, on rotation the fan or the rear turbine stages can touch the fan case, respectively the turbine housing. You can see it by irregular wear on the rub strip in the fan case.